The present invention relates generally to the preparation of the end of a bone to receive an implant, and more particularly to an instrument to facilitate the resection or shaping of the end of a bone.
In preparing a bone to receive an implant, it has been recognized that resection or shaping tools must be properly oriented with respect to the bone so that the implant to be secured to the resected or shaped surface of the bone fits properly and can provide the requisite support, freedom of movement, durability, stability, etc. While this is true of implants for virtually every joint of the human body, particular attention has been dedicated to the knee joint since the knee joint must be capable of supporting the weight of the human body and withstanding substantial shock and pounding, as well as having freedom of movement.
In preparing a knee joint to receive a total knee implant, the distal end of the femoral bone and the proximal end of the tibial bone must be prepared to receive the components of the knee implant. Since proper orientation and fit of the knee implant will provide for the best performance for the knee implant, the end of the femoral and tibial bones must, in most cases, be resected, shaped, or otherwise prepared with reference to the skeletal arrangement of the bones, the size and shape of the bones and the condylar surfaces, etc. Thus, the cutting or shaping guides used to facilitate the shaping of the end of the femoral and tibial bones must be properly oriented and secured to provide accurate and reproducible cuts. Such cutting or shaping guides are often secured on rods or other means which provide proper alignment and/or orientation.
With regard to the preparation of the distal end of the femoral bone, it is necessary to properly orient the cutting block or other guide with respect to the mechanical axis of the femoral bone, the mechanical axis being defined by a line drawn between the center of the femoral head, which sits in the hip socket, and the center of the intercondylar notch (between the condyles at the distal end of the femoral bone). In order to prepare the distal end of the femoral bone so that the knee prosthesis provides the requisite support, freedom of movement, durability, stability, etc., the cutting block must be oriented so that the distal cut at the distal end of the femoral bone is 90.degree. to the mechanical axis of the femoral bone. Thus, it has been recognized that the mechanical axis of the femoral bone must be identified or otherwise taken into account prior to preparing the distal end of the femoral bone.
One method of properly orienting a cutting block or guide at the distal end of the femoral bone is to utilize preoperative and/or intraoperative radiographs to facilitate the identification of the mechanical axis. Another method, which might be used in conjunction with radiographs, is to use an external alignment rod which is placed in line with the femoral head and the center of the intercondylar notch. One of the shortcomings of this method is that the femoral head is difficult to locate externally.
It has also been recognized that the anatomical axis of the femoral bone, defined by a line beginning in the intercondylar notch and extending along the center of the femoral shaft, has a relationship to the mechanical axis of the femoral bone. In particular, because of the cantilever arrangement of the femoral head, the anatomical axis of the femoral bone is offset from the mechanical axis of the femoral bone by a certain degree, typically having a magnitude of 5.degree. to 8.degree., though it is recognized that the angle of the offset may be outside of this range for particularly short or particularly tall individuals or due to a deformity in the femoral bone.
In view of the relationship between the anatomical axis and the mechanical axis of the femoral bone, it naturally followed that rather than identifying the mechanical axis of the femoral bone, the mechanical axis can simply be taken into account by utilizing the anatomical axis of the femoral bone, which can be approximated more easily than the mechanical axis. One method of properly orienting a cutting block or guide to make the distal cut on the femoral bone is to utilize a long external alignment rod whereby the surgeon places the external alignment rod parallel to the femoral shaft in both the anterior-posterior and medial-lateral planes. The surgeon would then know approximately where the mechanical axis of the femoral bone would lie so that the cutting block or guide could be arranged, with the appropriate angular adjustment, to make the distal cut with respect to the approximated mechanical axis. Of course, since the bone itself could not be seen during external orientation of the alignment rod with the femoral shaft, and particularly when operating on obese patients, there may be some error in properly orienting the cutting block or guide, or the holding means therefor. The use of an intraoperative radiograph may improve such external orientation, but the surgical procedure then becomes more time-consuming, tedious, and is still subject to error.
Accordingly, it was recognized that the use of an intramedullary alignment rod inserted into the medullary canal of the bone could be used to take into account the mechanical axis of the femoral bone without having to identify such mechanical axis beyond a preoperative radiograph to determine or approximate the angle between the mechanical axis and the anatomical axis. Such an intramedullary alignment rod is a properly oriented means for holding the cutting guide. Of course, some surgeons may still choose to use any one or both of the above-described methods of taking intraoperative radiographs and using an external alignment rod to check alignment of the cutting guide and/or the intramedullary rod.
The use of intramedullary alignment rods has been widespread. Examples of such intramedullary alignment rods are that which is disclosed in U.S. Pat. No. 4,487,203 to Androphy, the IM rod of the Multiflex Total Knee System by the Surgical Products Division of 3M Company, the IM rod of the R.M.C. Total Knee System by Richards Manufacturing Co., Inc. and that which is disclosed in the paper entitled, "Knee Replacement Using the Insall/Burstein Total Condylar Knee System" by John Insall, M.D. and Albert H. Burstein, Ph.D. for the New York Society for the Relief of the Ruptured and Crippled. As the above-cited examples reveal, several different expedients have been used in connection with intramedullary alignment rods to orient the cutting guide with respect to the mechanical axis of the femoral bone. For instance, in the Androphy patent, the intramedullary rod includes a long portion which extends into the medullary canal of the femoral bone and a short external portion which extends from the long portion at a 90.degree. angle and on which a cutting block can be provided. The cutting block on the short external portion can then be rotated so that the cutting block is oriented to guide the distal cut at 90.degree. to the mechanical axis of the femoral bone. In the 3M Multiflex Total Knee System, the rod is bent at a preselected angle so that the bent portion which extends externally of the femoral bone is approximately in line with the mechanical axis of the femoral bone. A cutting and/or drilling guide can then be placed on the bent portion so that a distal cut guide surface on the cutting block is 90.degree. to the bent portion, and consequently to the mechanical axis. The rod of the R.M.C. Total Knee System is similar to that of the 3M Multiflex System. In the article regarding the Insall/Burstein Total Condylar Knee System, a long IM rod is shown and discussed for use in the tibia where the rod extends from the proximal end almost to the ankle. In addition to the above systems, the use of a straight alignment rod has also been proposed whereby the hole in the cutting block which is to be supported on the straight alignment rod is bored at a preselected angle to the distal cut guide surface, and all other guide surfaces on the cutting block are referenced from the distal cut guide surface.
Because of the anatomical shape of the medullary canal of the femoral bone, tapering from wide to narrow and having a degree of curvature, those in the art have considered the use of longer intramedullary alignment rods to more accurately approximate the anatomical axis of the femoral bone. In a paper entitled, "Alignment of Total Knee Components", by Richard S. Laskin, M.D., published in the January 1984 issue of Orthopedics, Dr. Laskin describes his experience with various alignment systems. In this paper, Dr. Laskin not only concluded that the degree of accuracy in utilizing an intramedullary alignment rod far surpassed the degree of accuracy obtained by using either the method of identifying the mechanical axis by locating the femoral head or the use of the external alignment rod where the external aspect of the femoral shaft is used for reference, but also concluded that a longer intramedullary alignment rod provided more accurate results than a shorter intramedullary alignment rod.
Since the femoral canal tapers from a relatively wide portion at the distal end to the isthmus, typically considered to be the narrowest portion of the medullary canal in cross-section, though it may vary, and since the anatomical axis of a femoral bone is believed to pass along the center of the femoral shaft, it has been postulated that the isthmus provides the most accurate approximation of the anatomical axis of the femoral bone; even though the isthmus itself is curved in many cases, and it is difficult to precisely define the same. However, even at the time of the 3M Multiflex System and the Richards R.M.C. System, it was thought that an intramedullary alignment rod which approaches the isthmus, which in most cases will have a length and is not a single point in the canal, and does not necessarily extend through the isthmus would suffice to provide proper orientation of the intramedullary alignment rod for supporting a cutting guide in appropriate relation to the mechanical axis, as taken into account by the approximation of the anatomical axis of the femoral bone. If there were any error in the approximation of the anatomical axis by reason of the rod not extending into and/or through the isthmus, such error would be considered insignificant in properly orienting the cutting block with respect to the mechanical axis; especially since even rods which do extend into and/or through the isthmus might result in error for a variety of reasons, including the rod did not extend through the center of the canal or the canal itself simply did not provide the best approximately of the anatomical axis.
Others have taken the approach that to provide the most accurate approximation of the anatomical axis of the femoral bone, it is necessary not only to extend into and through the isthmus, but also to provide a close fit within the isthmus. Thus, in U.S. Pat. No. 4,474,177 to Whiteside, a method is taught whereby the surgeon is to first ream the medullary canal with a reamer having a diameter which approximates the diameter of the isthmus, followed by the use of an intramedullary alignment rod which has an extramedullary portion which is set at a preselected angle to the intramedullary portion. The intramedullary portion must be long enough to extend into and through the isthmus, and must have a large enough diameter (larger than the reamer) such that it is bound by the cortical bone which forms the isthmus. Accordingly, an "interference fit" results to purportedly provide the proper orientation as well as stability of the alignment rod.
The interference fit taught in the Whiteside patent is undesirable since it requires a reaming step in which a reamer is inserted past the isthmus of the femoral bone such that the axis of the reamer hopefully coincides with the anatomical axis of the femoral bone. The use of the reamer is intended to follow the normally curved medullary canal of the femoral bone and cut any portions of the cortical bone which might be in the way. Such reaming is not only time consuming and requires additional effort on the part of the surgeon, but more importantly, it causes a disturbance in the medullary canal and the cortical bone surrounding the medullary canal. In fact, such an interference fit of the rod might cause a fat embolism which even fluting on the rod could not prevent. Still further, the reamer used in the procedure taught by Whiteside could very well cut into the cortical bone surrounding the canal, thereby possibly redefining the shape of the canal and possibly the isthmus of the canal. The alignment rod in Whiteside is fit into the medullary canal after the reamer is removed, the alignment rod being bound by cortical bone. One must also be careful in utilizing the rod which is larger than that which has been reamed since too tight a fit in the canal could cause the bone to shatter upon insertion of the rod.
The present invention obviates the above shortcomings while providing a rigid alignment rod which facilitates proper orientation and stability of a tool guide or tool for preparing the end of a bone.